I have two female bettas that don't ever nip each other together in a 10 gallon tank. I am looking to get them some tankmates, and am debating between adding a couple mollies/platties/tetras or a small group of danios/guppies. Really what I'm looking for are some personal opinions and stories about which you think would look the best. Thanks so much! =D

Three more females. Having two females in a tank together will almost certainly not be successful long term. If I were you I would buy three more females, quarantine them for at least two weeks, and then introduce them to the tank.

I agree with Matt. I know it may seem like they're fine now and that they have been for a year but it won't be pretty the moment they do snap.

Either way though, if you add other fish, it's going to stress them out more and possibly cause them to get aggressive. So either keep them how they are or get more females, that's my suggestion is all

Danios also thrive in cooler water than bettas, and are super fast, which can stress a betta out.
I would suggest looking at your water parameters. Hard water with a high pH will suit livebearers; soft water with a low pH would be better for tetras, kuhlis or cories.